1. Field of the Invention
The present invention relates to an image reading apparatus for photoelectrically reading an image and, more particularly, to an image reading apparatus using a plurality of image sensors.
2. Description of the Prior Art
Among conventional image reading apparatuses of this type, an image reading apparatus has been proposed wherein a plurality of CCD image sensor arrays each having a plurality of light-receiving elements are aligned along a main scan direction. In this image reading apparatus, unless the CCDs are properly aligned, the joint portions between every two adjacent CCDs have an overlapping portion or a portion not sensed by either CCD (an omitted portion). As a result, when a read image is reproduced, a stripe is formed at a portion corresponding to the joint of every two adjacent CCDs along a subscan direction, resulting in poor image reproduction.
A pixel pitch of the CCD is as small as several tens of microns, so it is difficult to align the CCDs with high precision. Properly aligned CCDs may be misaligned due to a mechanical impact when the image reading apparatus is moved from one place to another.
In order to prevent this, CCDs are conventionally aligned in the following manner.
The CCDs are aligned along the main scan direction such that image reading regions thereof partially overlap. Marks are formed at the overlap portions excluding the effective image reading region of the plurality of CCDs arranged as described above. These marks are read by the CCDs before scanning. As a result, an identical mark is read by two adjacent CCDs whose image reading regions overlap, so that the image signals therefrom can be used to calculate how many bits overlap. An image signal representing the overlapped bit component is electrically subtracted from each one-line image signal obtained by scanning an original. Therefore, image joint correction of the plurality of CCDs can be automatically performed.
However, according to this conventional automatic joint correction technique, the two CCDs must be aligned so that the amount of their overlap is in a predetermined range (e.g., 128 bits, i.e., 8 mm at a resolution of 16 pixels/mm). Such alignment is easier than aligning the CCDs along the main scan direction without overlapping or omission. However, high precision is still required in units of several millimeters. In addition to this disadvantage, when automatic joint correction cannot be performed due to misalignment of CCDs which is caused by a mechanical impact or the like while the image reading apparatus is being operated, the direction along which the CCDs are shifted is unknown. In addition, the displacement of the CCDs is also unknown. In order to determine the direction and the displacement of the CCDs, measuring equipment such as an oscilloscope is required, resulting in inconvenience. Correspondence between the respective bits of the CCDs and the reading locations cannot be accurately detected. For this reason, problems are presented, mainly in assembly and maintenance.
Furthermore, according to this conventional technique, when a plane with a joint mark is not optically aligned with an original surface, joint correction cannot be properly performed.
The focal point of the CCD is aligned with the original surface, so that the joint mark is read as an image which is out of focus. As a result, even if a corrected value (the number of bits of the overlap portion of the two adjacent CCDs) is calculated by an image signal representing the read joint mark, a desired joint correction cannot be performed, resulting in poor image reproduction.